In a first, public, mode of a display device that is switchable between a public and private display mode, the device commonly behaves as a standard display. A single image is displayed by the device to as wide a viewing angle range as possible, with optimum brightness, image contrast and resolution for all viewers. In the second, private mode, the main image is discernible only from within a reduced range of viewing angles, usually centred on the normal to the display surface. Viewers regarding the display from outside this reduced angular range will perceive either a second, masking image which obscures the main image, or a main image so degraded as to render it unintelligible.
This concept is illustrated in FIG. 4 of the accompanying drawings, in which a “main view” 402 of a display device 401 is visible substantially only to the principal user 404 of the display device 401 when his or her eye is close to the principal axis of the display device 401, and one or more “side views” 403 designed to be visible to other users 405 assumed to be off-axis.
This concept can be applied to many devices where a user may benefit from the option of a privacy function on their normally wide-view display, for use in certain public situations where privacy is desirable. Examples of such devices include mobile phones, Personal Digital Assistants (PDAs), laptop computers, desktop monitors, Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) equipment. Such devices can also be beneficial in situations where it is distracting and therefore unsafe for certain viewers (for example drivers or those operating heavy machinery) to be able to see certain images at certain times, for example an in car television screen while the car is in motion.
Several methods exist for adding a light controlling apparatus to a naturally wide-viewing range display:
One such structure for controlling the direction of light is a ‘louvred’ film. The film consists of alternating transparent and opaque layers in an arrangement similar to a Venetian blind. Like a Venetian blind, it allows light to pass through it when the light is travelling in a direction nearly parallel to the layers, but absorbs light travelling at large angles to the plane of the layers. These layers may be perpendicular to the surface of the film or at some other angle. Methods for the production of such films are described in a USRE27617 (F. O. Olsen; 3M 1973), U.S. Pat. No. 4,766,023 (S.-L. Lu, 3M 1988), and U.S. Pat. No. 4,764,410 (R. F. Grzywinski; 3M 1988).
Other methods exist for making films with similar properties to the louvred film. These are described, for example, in US05147716 (P. A. Bellus; 3M 1992), and US05528319 (R. R. Austin; Photran Corp. 1996).
Louvre films may be placed either in front of a display panel or between a transmissive display and its backlight to restrict the range of angles from which the display can be viewed. In other words, they make a display “private”.
The principal limitation of such films is that they require mechanical manipulation, i.e. removal of the film, to change the display between the public and private viewing modes:
In GB2413394 (Sharp, 2004), an electronically switchable privacy device is constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way. Devices utilising this technology include the Sharp Sh851i and Sh902i mobile phones.
The above methods suffer the disadvantage that they require the addition of extra apparatus to the display to provide the functionality of electrically switching the viewing angle range. This adds cost, and particularly bulk to the display, which is very undesirable, particularly in mobile display applications such as mobile phones and laptop computers.
Methods to control the viewing angle properties of an LCD by switching the single liquid crystal layer of the display between two different configurations, both of which are capable of displaying a high quality image to the on-axis viewer are described in US20070040780A1 (Sharp, 2005) and WO2009057417A1 (Sharp, 2007). These devices provide the switchable privacy function without the need for added display thickness, but require complex pixel electrode designs and other manufacturing modifications to a standard display.
An example of a display device with privacy mode capability with no added display hardware complexity is disclosed in WO 2009/069048. Another such example is provided in US20090079674A1, which discloses a privacy mode for a display in which different levels of signal voltage are applied to adjacent pixels so that an averaged brightness of those pixels varies with the signal voltages according to the display's gamma curve to show an expected image when viewed on axis, and in which the averaged brightness is at a constant level within a specified voltage range when viewed off axis, so as to change a contrast of the image to a visibly unidentifiable degree off axis.
Another example of a display device with privacy mode capability with no added display hardware complexity is the Sharp Sh702iS mobile phone. This uses a manipulation of the image data displayed on the phone's LCD, in conjunction with the angular data-luminance properties inherent to the liquid crystal mode used in the display, to produce a private mode in which the displayed information is unintelligible to viewers observing the display from an off-centre position. However, the quality of the image displayed to the legitimate, on-axis viewer in the private mode is severely degraded.
Similar schemes to that used on the Sh702iS phone, but which manipulate the image data in a manner dependent on a second, masking, image, and therefore causes that masking image to be perceived by the off-axis viewer when the modified image is displayed, are given in GB2428152A1 (published on 17 January 2007) and GB application GB0804022.2 (published as GB2457106A on 5 Aug. 2009). The method disclosed in the above publications uses the change in data value to luminance curve with viewing angle inherent in many liquid crystal display modes such as “Advanced Super View” (ASV) (IDW'02 Digest, pp 203-206) or Polymer Stabilised Alignment (PSA) (SID'04 Digest, pp 1200-1203).
The data values of the image displayed on the LC panel are altered in such a way that the modifications applied to neighbouring pixels effectively cancel out when viewed from the front of the display (on-axis), such that the main image is reproduced, but when viewed from an oblique (off-axis) angle, the modifications to neighbouring pixels result in a net luminance change, dependent on the degree of modification applied, so the perceived image may be altered.
It is desirable to provide improvements to the method described in GB2428152A1 and GB2457106A.